Heavy-duty rock bits are employed for drilling wells in subterranean formations for oil, gas, geothermal steam, and the like. Such bits have a body connected to a drill string and a plurality, typically three, of hollow cutter cones mounted on the body for drilling rock formations. The cutter cones are mounted on steel journals or pins integral with the bit body at its lower end. In use, the drill string and bit body are rotated in the bore hole, and each cone is caused to rotate on its respective journal as the cone contacts the bottom of the bore hole being drilled.
While such a rock bit is used in hard, tough formations, high pressures and temperatures are encountered. The total useful life of a rock bit in such severe environments is in the order of 20 to 200 hours for bits in sizes of about 3¾ to 36 inch diameter at depths of about 5000 to 20,000 feet. Useful lifetimes of about 65 to 150 hours are typical. When a rock bit wears out or fails as a bore hole is being drilled, it is necessary to withdraw the drill string for replacing the bit. Prolonging the time of drilling minimizes the lost time in “round tripping” the drill string for replacing bits. Replacement of a drill bit can be required for a number of reasons, including wearing out or breakage of the structure contacting the rock formation. One reason for replacing the rock bits includes failure or severe wear of the journal bearings on which the cutter cones are mounted. The journal bearings are lubricated with grease adapted to severe conditions. Lubrication failure can sometimes be attributed to misfit of bearings or seal failure, as well as problems with a grease.
The journal bearings are subjected to very high pressure drilling loads, high hydrostatic pressures in the hole being drilled, and high temperatures due to drilling as well as elevated temperatures in the formation being drilled. Considerable development work has been conducted over the years to produce bearing structures and employ materials that minimize wear and failure of such bearings.
A variety of grease compositions have been employed in the past. For example, one suitable grease composition includes a generally low viscosity, refined petroleum or hydrocarbon oil basestock which provides the basic lubricity of the composition and may constitute about ¾ of the total grease composition. Such basestock oil is thickened with a conventional metal soap or metal complex soap wherein the metal is aluminum, barium, calcium, lithium, sodium, or strontium. U.S. Pat. No. 4,358,384 discloses such a grease composition including a petroleum derived mineral oil lubricant basestock and a metal soap or metal complex soap including aluminum, barium, calcium, lithium, sodium or strontium metals.
In order to enhance the film lubricating capacity of such petroleum basestock greases, solid additives such as molybdenum disulfide, copper, lead or graphite may be added. Synthetic polymer extreme pressure agents (EPA) are also used. Such additives serve to enhance the ability of the lubricant basestock to form a film between the moving metal surfaces under conditions of extreme pressure. U.S. Pat. Nos. 4,358,384, 3,062,741, 3,107,878, 3,281,355, and 3,384,582 disclose the use of molybdenum disulfide and other solid additives such as copper, lead, silver and graphite which have been employed to attempt to enhance the lubrication properties of oils and greases. It is also known to include metallic oxides like zinc oxide or lead oxide in lubrication oils. Not all of these and other additives are suitable for use in grease compositions for the extreme conditions encountered by a rock bit. Copper and lead, for example, are now considered undesirable if they can be avoided.
It is, therefore, desirable to provide a grease composition for lubricating rock bits that protects the journal bearing surfaces from premature wear or failure during service at the high temperatures, bearing pressures and rotational speeds often found in modern rock bits. It is also desirable that the grease composition complement the sealing arrangement and promote optimum sealing.